Semiconductor integrated circuits are produced by a plurality of processes in a wafer fabrication facility (fab). These processes, and associated fabrication tools, may include thermal oxidation, diffusion, ion implantation, RTP (rapid thermal processing), CVD (chemical vapor deposition), PVD (physical vapor deposition), epitaxy, etch, and photolithography. During the fabrication stages, semiconductor wafers are monitored for quality assurance and yield using various metrology tools. As feature sizes are reduced, the amount of monitoring and controlling may need to be increased. This, however, increases costs by the increased quantity of metrology tools required, the increased manpower to perform the monitoring and controlling, and the associated delay in manufacturing cycle time. Computer Integrated Manufacturing (CIM) is an emerging technology that utilizes virtual metrology for wafer acceptance testing (WAT). Virtual metrology is a technique that predicts results of a semiconductor process based on modeling. As semiconductor fabrication progresses to advanced technology node processes (e.g., 90 nm to 65 nm to 45 nm to 32 nm), the prediction error of virtual metrology needs to be reduced to satisfy tighter control specifications.